Connected: An Internet Encyclopedia
RFC 1771 - 9.1.2 Phase 2: Route Selection

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### 9.1.2 Phase 2: Route Selection

The Phase 2 decision function shall be invoked on completion of Phase 1. The Phase 2 function is a separate process which completes when it has no further work to do. The Phase 2 process shall consider all routes that are present in the Adj-RIBs-In, including those received from BGP speakers located in its own autonomous system and those received from BGP speakers located in neighboring autonomous systems.

The Phase 2 decision function shall be blocked from running while the Phase 3 decision function is in process. The Phase 2 function shall lock all Adj-RIBs-In prior to commencing its function, and shall unlock them on completion.

If the NEXT_HOP attribute of a BGP route depicts an address to which the local BGP speaker doesn't have a route in its Loc-RIB, the BGP route SHOULD be excluded from the Phase 2 decision function.

For each set of destinations for which a feasible route exists in the Adj-RIBs-In, the local BGP speaker shall identify the route that has:

1. the highest degree of preference of any route to the same set of destinations, or

2. is the only route to that destination, or

3. is selected as a result of the Phase 2 tie breaking rules specified in 9.1.2.1.

The local speaker SHALL then install that route in the Loc-RIB, replacing any route to the same destination that is currently being held in the Loc-RIB. The local speaker MUST determine the immediate next hop to the address depicted by the NEXT_HOP attribute of the selected route by performing a lookup in the IGP and selecting one of the possible paths in the IGP. This immediate next hop MUST be used when installing the selected route in the Loc-RIB. If the route to the address depicted by the NEXT_HOP attribute changes such that the immediate next hop changes, route selection should be recalculated as specified above.

Unfeasible routes shall be removed from the Loc-RIB, and corresponding unfeasible routes shall then be removed from the Adj- RIBs-In.

9.1.2.1 Breaking Ties (Phase 2)

Next: 9.1.2.1 Breaking Ties (Phase 2)

Connected: An Internet Encyclopedia
RFC 1771 - 9.1.2 Phase 2: Route Selection